Mycobacterium tuberculosis (Mtb) is the most common bacterial pathogen in humans and a major global health threat, responsible for 9 million new cases and 1.5 million deaths in 2013. Roughly 10% of Mtb infections result in unrestrained bacterial proliferation and active disease. In the other 90%, the immune system controls Mtb infection but usually cannot eradicate it fully: Mtb persist within the host and can be reactivated when the immune system weakens. The host-pathogen interplay that determines the course of infection and the effectiveness of immune control of Mtb is still very poorly understood. Our recent work has focused on the role of iron in Mtb pathogenesis. Iron (Fe) is an essential micronutrient influencing both Mtb proliferation and immune system function. Mtb, like other successful pathogens, has evolved multiple mechanisms to acquire Fe from the host and to adjust to changes in Fe availability. Iron restriction inhibits bacterial replication, while a more iron-rich environment is conducive to Mtb proliferation and reactivation of latent TB. Additionally, macrophage-mediated immunity and inflammation have been linked with iron metabolism by reports showing that anti-inflammatory and pro-inflammatory cytokine activated macrophages have distinct patterns of iron metabolism gene expression and handle iron differently. Based on these and other observations, we hypothesize that the iron environment of TB lesions and individual macrophages is a determining factor in Mtb infection. A hallmark of TB is the granuloma ? an organized aggregate of immune cells surrounding Mtb-infected macrophages ? which can either serve as an effective barrier to prevent Mtb spread or can act as a source of bacilli during active disease. We propose to characterize the iron microenvironment of individual rabbit granulomas to understand its impact on Mtb-host interplay. The rabbit model of Mtb infection can produce either active disease or latency; both models lead to formation of lung granulomas, which recapitulate key histopathological features of human granulomas. We have at our disposal a collection of pathologically diverse granulomas from infected rabbit lungs. We will use LCM coupled to ICP-MS and immunohistochemistry coupled to confocal microscopy techniques to characterize these granulomas in terms of iron distribution, macrophage type, bacillary load, and overall granuloma architecture. These studies will be complemented by ex vivo analyses of macrophage populations derived from fresh granuloma samples. We will also analyze data from transcriptome analysis of infected rabbit lungs for specific gene expression patterns in iron metabolism pathways. The results of this study will set the foundation for mechanistic studies that can potentially reveal targets for new pathogen and/or host directed therapies.